1. Field of the Invention
This invention relates to a switching system adapted to perform switching processings of voice, data, image information and the like and more particularly to a time-division channel arrangement suitable for performing switching of information, in a unit of a block accommodating communication control information, between switching nodes or modules in a star-type configuration.
2. Description of the Related Art
In the field of switching systems, studies have concentrated on the realization of high-speed and wide band performance, increase in the capacity of the channel and advanced improvements in reliability. Conceivably, a specified expedient to this end resides in the decentralization of a channel system, especially, a multiplexer stage. This expedient takes advantage of the decentralization of load which contributes to improvement in throughput increase in the capacity of the overall exchange and risk-decentralization owing to the decentralization of the channel system.
In the past, the exchange has played a major role in switching low-speed telephone voice, but it is expected that high-speed data communications as applicable to image data will be in great demand in future. Under the circumstances, the exchange is required to be drastically improved in call throughput but throughput of a processor can not be extended unlimitedly. To promote the throughput of the processor, the distribution of function and the distribution of load based on a multi-processor scheme may conceivably be adopted. This countermeasure, however, invites complexity of software and besides suffers from a bottleneck caused by communications between processors and peripherals such as magnetic disks, printers, etc., thus failing to eliminate the limitation imposed on the throughput.
Incidentally, from the standpoint of the enlargement of the system and the decentralization of risks such as system break down, call congestion or intended or unintended destruction of a switching system, it is desirable that individual function units of the exchange be realized with individual modules and these modules be physically distributed. In such an instance, unless an independent distribution configuration is adopted wherein processors per se are distributed, the load will be concentrated on a central processor and inconveniently the number of lines for controlling each distributed module will be increased.
In a distributed type switching system in which the speech path system is distributed and the control system is concentrated at a location, throughput of the control system is limited and the merits of the distribution of load can not fully mature to advantage. Under the circumstances, a foreseen switching system is desired to be of an independent distributed configuration in which not only the speech path system but also the control system is distributed. The use of independent modules is however problematic in that when a module communicates with another module, the sending module has to know whether a channel to the partner module is idle and whether an outgoing circuit from the partner module is idle. In other words, resource management is needed. Even in the ordinary distributed system, resource management is often concentrated at a location and all modules interrogate a common managing unit. Consequently, a bottleneck of processing is caused by the managing unit, especially, in a large-scale system. On the other hand, the system of full independent distributed modules is forced to take either a way to enable one module to constantly know the status of all of the other modules or a way to permit one module to confirm the status of the partner module each time a call is set up. In the former way, one module, when its status changes, is required to inform all remaining modules of the change or all of the modules must mutually confirm their status periodically. Even with the above procedure completed, when a module has only one idle circuit, there is a possibility that the remaining modules will transmit communication requests to that module at the same time. In the latter way, the above problems are not encountered but the necessity of mutual communications among all of the modules is by itself problematic. As a countermeasure, it is conceivable to establish communication lines in a mesh configuration among the modules. However, this becomes costly due to the complicated physical geometry and due to the fact that additional management of communication is needed.
Incidentally, in recent years, the trend of integratedly communicating various kinds of information including voice and data has become active. This trend originates from a desire for efficiency and economization but realization of the integrated communications needs integrated switching processings of various kinds of information. In particular, it is desired that switching of circuit switching information such as voice of which real-time base processings are required and switching of storable data or storage switching information which has hitherto been handled by a packet exchange can both be effected integratedly through the same channel.
A time-division channel arrangement for integratedly handling voice and data, that is, a so-called integrated channel arrangement is known as disclosed in JP-A-61-60044 entitled xe2x80x9cBlock switching systemxe2x80x9d and JP-A-60-127844 entitled xe2x80x9cCircuit/packet integrated switching systemxe2x80x9d. In the former literature, also as disclosed in Proceedings of International Switching Symposium 1987 SESSION B. 7.1 xe2x80x9cELASTIC BASKET SWITCHINGxe2x80x94A NEW INTEGRATED SWITCHING SYSTEM FOR VOICE AND HIGH-SPEED BURST DATAxe2x80x94xe2x80x9d by S. Morita et al, a boundary identifier indicative of the boundary between communication channels is dynamically set in accordance with the amount of information in a set-up call in order that either of information requiring real-time base processing and information requiring burst base processing can be switched one-dimensionally. In the latter literature, also as disclosed in Proceedings of International Switching Symposium 1984 SESSION 42B-3 xe2x80x9cSYNCHRONOUS COMPOSITE PACKET SWITCHING FOR ISDN SWITCHING SYSTEM ARCHITECTURExe2x80x9d by T. Takeuchi et al, a plurality of distributed communication nodes are connected in loop, the communication node being a switching module having a subscriber interface or a trunk interface, and each switching module packets circuit switching information such as voice and packet switching information such as data in blocks of fixed length in accordance with destination switching modules and adds a destination node number to each block, so that the information may be switched through the loop channel.
In the aforementioned xe2x80x9cBlock switching systemxe2x80x9d, the time switch function of the channel can efficiently be realized but because of each block being of a variable length, the space switch function of the channel for mutually rearranging blocks on a highway is difficult to achieve. Accordingly, this system is unsuitable for a large-scale exchange.
In the aforementioned xe2x80x9cCircuit/packet integrated switching systemxe2x80x9d, on the other hand, all of the switching modules access the loop and in order to prevent the overall throughput from being decreased, the loop must be operated at so high a speed that the total of amounts of throughput of the individual modules can be handled by the loop. Accordingly, the larger the scale of the exchange, the more the high-speed device will be used in the interface between each switching module, and the loop becomes expensive.
In order to solve these problems, it is effective to use a block of fixed length which accommodates connection information and to construct a self-routing network which does not use a loop. However, while the use of the loop is convenient in that all the blocks carried on the loop can be processed sequentially with ease, a plurality of blocks used in, for example, the star-type configuration must be rearranged to prevent them from colliding with each other at a time within the central node. To this end, avoidance of concentration of individual blocks on a specified connection destination (outgoing highway) is necessary and blocks once stored must be controlled for switching such that they do not collide with each other, by monitoring destinations of individual blocks. However, in the case of switching communication information, intensively bursty, as appearing in a kind of data communications (for example, image information transfer), many blocks must be sent to the same destination within a short period of time but in the case of switching information requiring real-time base processing, such as voice, storage of blocks must be avoided as far as possible.
Reference may be made to JP-A-59-23658, U.S. Pat. No. 4,494,230 and International Publication No. W085/02735 (corresponding to JP-A-60-501833).
An object of the present invention is to provide an economical and high-throughput switching system.
Another object of the invention is to provide a highly reliable switching system based on the distribution of load and the decentralization of risks.
Still another object of the invention is to provide a time-division channel arrangement capable of meeting both the requests for real-time base processing and burst base processing.
According to the invention, to accomplish the above objects, a switching system comprises a plurality of front-end modules each adapted to perform a switching processing in association with a subscriber line or a trunk line, and a single or a plurality of central modules for interconnecting the plurality of front-end modules in star-type fashion via highways and switching information prevailing between the front-end modules, in a unit of a block accommodating the information with a header added thereto to contain connection control information and in accordance with the contents of the header.
Fundamentally, each of the front-end modules functions to determine a destination path for communication information coming from a subscriber line or a trunk line. Specifically, with the use of each intermodule highway having frames occurring at a predetermined period and a plurality of time slots contained in each frame the number of which is determined in predeterminedly ruled relationship with the number of subscriber line or trunk lines accommodated in the front-end module, each front-end module functions to transmit to the highway a block which accommodates the communication information and a header containing a destination front-end module number by loading the block on the time slots, functions to manage the status of the subscriber line or trunk line by constantly storing an idle/busy status thereof, functions to decide the idle/busy status of the subscriber line or trunk line, and functions to transmit and receive a result of the decision between the plurality of front-end modules.
The central module comprises a plurality of time switches connected to the front-end modules via the highways, a space switch connected to the time switches, a first status managing memory for monitoring every frame an idle/busy status of a link in association with an incoming highway of the space switch, a second status managing memory for monitoring in every frame an idle/busy status of a link in association with an outgoing highway of the space switch, and a circuit for generating a read address or a write address for each of the plurality of time switches by looking up the first and second status managing memories, in such a manner that a plurality of blocks having the same destination are not switched at the same time.
Further, in the central module of the invention, each incoming highway associated with each incoming front-end module has branches for the purpose of handling both the circuit switching information and packet switching information, one branch being directly connected to the time switch and the other being connected to a storage circuit, and the storage circuit is selectively used.
By selectively using the storage circuit, the communication information such as voice of which the real-time base processing is required (circuit switching information) can directly be delivered from the incoming highway to the time switch without being passed through the storage circuit, so that real-time base processing can be completed. On the other hand, the storable communication information (storage switching information) is temporarily stored in the storage circuit, so that the storage switching information can be switched through the use of the same channel as used for the circuit switching information.
The present invention was disclosed by some of the present inventors in an article entitled xe2x80x9cA NEW DISTRIBUTED SWITCHING SYSTEM ARCHITECTURE FOR MEDIA INTEGRATIONxe2x80x9d published on Jun. 10, 1987, PROCEEDINGS 5 OF IEEE INTERNATIONAL CONFERENCE ""87 SESSION 11.4.